Vesicle systems, kinetic processes

The general field with which this review is concerned is currently one of the most exciting in chemical physics, the study of kinetic processes in systems of finite size and/or of restricted dimensionality. Problems ranging from the study of organized molecular assemblies (micelles, vesicles, microemulsions), biological systems (cells, microtubules, chloroplasts, mitochondria), structured media such as clays and zeolites, and nucleation phenomena in finite domains are among those under active investigation. 

Some inhibitors of the vesicle uptake process are listed in Table IS. The most potent inhibitors are reserpine and related substances. Kinetic studies indicate that reserpine has an affinity for the vesicle uptake sites some 10,000 times higher than that of the catecholamines. Another compound with very high affinity is prenylamine (segontin), which is about 10 times less potent than reserpine. Imi-pramine and chlorpromazine are both active as inhibitors, but are very much less potent on the vesicle system than on the neuronal membrane uptake. Other inhibitors include substances, such as N-ethylmaleimide, which block free sulphydryl groups. The action of such compounds appears to be related to an inhibition of the storage vesicle ATP-ase (Para. 5.2.4). 

Overall, these two techniques permit the determination of relaxation times in the range 0.5 ts to 0.3 ns. They have been used extensively at the early stage of kinetic studies of micellar solutions for the study of the exchange process of surfactants with a relatively short alkyl chain and, more recently, for the study of novel surfactants, including gemini (dimeric) surfactants. They have also been used for the study of phase transition in vesicle systems. 

The fact that the gaseoues reactants react very quickly means that, in practice and according to model B, the reaction takes place at the phase boundary or in an interfacial layer with a relatively small thickness [30, 32], The latter has been proven which - via process modeling on the basis of appropriate kinetic models -made possible a more optimal reactor and mixing design [43], Additionally, there is much (industrially initiated) work underway to check the addition of counterions or surface active ligands (Sections 3.2.4 and 3.2.6) or to test measures which increase the widths of the interfacial layers or the consequences of micelle/vesicle-forming devices (Section 4.5) [45]. The dependence of the reactivity of aqueous systems on the solubility of the reactants in the aqueous catalyst solutions is of appreciable importance for the problem of universal applicability (cf., e.g., Sections 4.1, 4.2, 6.1.3.2, and Chapter 7). 

In classical, continuum theories of diffusion-reaction processes based on a Fickian parabolic partial differential equation of the form, Eq. (4.1), specification of the Laplacian operator is required. Although this specification is immediate for spaces of integral dimension, it is less straightforward for spaces of intermediate or fractal dimension [47,55,56]. As examples of problems in chemical kinetics where the relevance of an approach based on Eq. (4.1) is open to question, one can cite the avalanche of work reported over the past two decades on diffusion-reaction processes in microheterogeneous media, as exemplified by the compartmentalized systems such as zeolites, clays and organized molecular assemblies such as micelles and vesicles (see below). In these systems, the (local) dimension of the diffusion space is often not clearly defined. 

Many studies investigated the kinetics of the process of vesicle break up upon addition of a micelle-forming surfactant and the reverse process of vesicle formation when a system containing a micelle-forming surfactant is... 

The methods has been used to study the kinetics of ionophore mediated transport across phopholipid vesicles. In this system all the parameters affecting the transport process (i.e. ion and ionophore concentration, lipid concentration and ccnqposition, pH cuid ten rature) were quantitatively controlled and studied, thus enabling to analyze the mechanism of the transport. 

One particular asset of structured self-assemblies is their ability to create nano- to microsized domains, snch as cavities, that could be exploited for chemical synthesis and catalysis. Many kinds of organized self-assemblies have been proved to act as efficient nanoreactors, and several chapters of this book discnss some of them such as small discrete supramolecular vessels (Chapter Reactivity In Nanoscale Vessels, Supramolecular Reactivity), dendrimers (Chapter Supramolecular Dendrlmer Chemistry, Soft Matter), or protein cages and virus capsids (Chapter Viruses as Self-Assembled Templates, Self-Processes). In this chapter, we focus on larger and softer self-assembled structures such as micelles, vesicles, liquid crystals (LCs), or gels, which are made of surfactants, block copolymers, or amphiphilic peptides. In addition, only the systems that present a high kinetic lability (i.e., dynamic) of their aggregated building blocks are considered more static objects such as most of polymersomes and molecularly imprinted polymers are discussed elsewhere (Chapters Assembly of Block Copolymers and Molecularly Imprinted Polymers, Soft Matter, respectively). Finally, for each of these dynamic systems, we describe their functional properties with respect to their potential for the promotion and catalysis of molecular and biomolecu-lar transformations, polymerization, self-replication, metal colloid formation, and mineralization processes. 

Alkaline hydrolysis of ethyl caprylate (itself insoluble in water) yields sodium caprylate, initially at a very slow rate bnt as soon as sufficient caprylate was formed for aggregation into micelles to take place, the authors observed an exponential increase in reaction rate owing to micellar catalysis. These self-assembling surfactant strucmres may consequently provide a model system for studies of pre-biotic chemistry. The possible relevance of this process to prebiotic chemistry was emphasized by their observation that the micelles can be converted into more robust vesicles by a pH change induced by dissolved CO2, and latter on, Luisi extended this approach to vesicular systems (see Section 3.3). Kinetic models for this kind of autocatalytic dynamic systems were also developed in the literature." ... 

Here we shall analyze the mechanisms and kinetic regularities of proton transfer photoreactions in the simple model systems - in micelles and vesicles from the point of view of classical chemical kinetics. The energetics and dynamics of chemical processes in microphases, and energetics and dynamics of diffusion steps in microphases and between them are considered. 

The book first discusses. self-assembling processes taking place in aqueous surfactant solutions and the dynamic character of surfactant self-assemblies. The next chapter reviews methods that permit the. study of the dynamics of self-assemblies. The dynamics of micelles of surfactants and block copolymers,. solubilized systems, microemulsions, vesicles, and lyotropic liquid crystals/mesophases are reviewed. successively. The authors point out the similarities and differences in the behavior of the.se different self-as.semblies. Much emphasis is put on the processes of surfactant exchange and of micelle formation/breakdown that determine the surfactant residence time in micelles, and the micelle lifetime. The la.st three chapters cover topics for which the dynamics of. surfactant self-assemblies can be important for a better understanding of observed behaviors dynamics of surfactant adsorption on surfaces, rheology of viscoelastic surfactant solutions, and kinetics of chemical reactions performed in surfactant self-assemblies used as microreactors. 

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